In a two-cycle engine, the entire cycle of internal combustion events (intake, compression, power, and exhaust) takes place in two piston strokes. Every other piston stroke (the down stroke) is a power stroke.
Uni-flow scavenged two-cycle engines are characterized by air intake at the bottom of the piston cylinder. The air intake port opens as the piston nears the bottom of its down stroke and closes soon after the up stroke begins. The air flows up toward the top of the cylinder during the up stroke and exits through exhaust valves at the top of the cylinder. These exhaust valves open just before the intake port opens during the down stroke, and close just after the air intake on the up stroke. The flow of air up through the cylinder "scavenges" the exhaust gas via the exhaust valves.
In their conventional configuration, uni-flow two-cycle engines, particularly those using homogeneous combustion, have a limited ability to run at reduced loads. Unlike four-stroke engines, they do not have separate intake and exhaust strokes. Due to the lack of an intake stroke, the intake air cannot be throttled because it is used to scavenge the exhaust gas. To reduce power, fuel flow can be reduced to a limited extent, but after a point (the lean flammability limit), reduced fuel flow will result in lack of combustion.